As I currently don't have a Soda Stream, I'd pay for sparkling tap water. I don't think sparkling water can explain all those percentage points, but maybe some of them?

That is actually just referring to regular old non-carbonated water, not sparkling water (or as we call it around here, seltzer).

One of the problems is water pipes which are largely untested. Soneven if you drink the same water, bottled water might still be safer. In theory. Practically I doubt it matters for most people.